CS8920 datasheet(48/66 Pages) CIRRUS

48 / 66 page

AN84REV1

CS8920 Technical Reference Manual

utilized by the CS8920 in its current

operational mode (e.g., IO mode only -- no

shared memory or DMA channel required).

Therefore, in order to avoid wasting ISA

resources and decrease the likelihood of an

unsuccessful autoconfiguration of the

CS8920 or other PnP device, the resource

descriptor for those resources not used

should be omitted from the resource

descriptor list.

One exception to this guideline concerns

the resource descriptors for the BootPROM

(Memory Descriptor 0) and shared memory

for memory-mapped access to the

PacketPage registers (Memory Descriptor

1). Memory Descriptor 1 will only be

processed if Memory Descriptor 0 is

defined. Therefore, if shared memory is

used (Memory Descriptor 1), a resource

descriptor for a BootPROM (Memory

Descriptor 0) must be placed in the

descriptor list regardless of whether or not a

BootPROM is used.

Note: Some PnP BIOSs have been observed

to ignore a memory descriptor with a range

length of zero bytes. Therefore, to provide

compatibility with as many BIOS versions

in the field as possible, yet keep the amount

of unused memory reserved by PnP to a

minimum, Crystal suggests using a 256-

byte range length value for Memory

Descriptor 0 when a BootPROM is not

used but shared memory is requested.

Figure 3.4.1 below is an example PnP

resource data record for a CS8920 ISA

Ethernet Adapter. This example assumes

the CS8920 will use IO space, shared

memory, IRQ, DMA, and a BootPROM is

installed. The record starts at EEPROM

word address 30h.

Refer to Section 6.2 of the Plug and Play

ISA Specification 1.0a for more

information on Plug and Play resource data

types.

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